1. Field of the Invention
The invention is related to the field of electric heaters and in particular to a glow plug having a conductive film heater for assisting the start ignition of a Diesel type engine.
2. Prior Art
Electrically energized glow plugs are currently used in compression ignited or Diesel type engines to assist in the ignition of the air/fuel mixture during cold starts. In particular glow plugs are essential in the northern states during the winter months when the ambient temperature falls below 10.degree. C. The function of the glow plug is to heat the air/fuel mixture to a temperature which will cause spontaneous combustion when compressed in the engine's cylinders. After the engine has started, the heat developed by the combustion of the air/fuel mixture will maintain the combustion chamber at a temperature sufficient to sustain continued operation of the engine and the electrical power to the glow plug can be terminated.
Initially, the heating element of the glow plug was a coiled resistance wire such as disclosed by Dorner in U.S. Pat. No. 1,957,762. However, the resistance wire would become brittle after a relatively short period of time and break off. Therefore these glow plugs had to be replaced frequently. Additionally the loose pieces of wire in the cylinders often became trapped between the piston and the cylinder wall and scored the cylinder walls causing permanent damage to the engine. To overcome these problems the prior art teaches enclosing the heater element in a cylindrical metal shield as taught by Kauhl et al in U.S. Pat. No. 4,200,077, Steinke in U.S. Pat. No. 4,252,091, or Mann in U.S. Pat. No. 4,281,451. Although these latter glow plugs have overcome the problems encountered with the bare wire glow plugs, their thermal response time is significantly increased. For example, there is currently about a 30 second delay from the time the glow plug is energized before the engine can be started. Additionally, thermal run-away of the heater element can melt the protective metal shield. Often the melted protective shield becomes so distorted that it is necessary to tear down the engine to remove the failed glow plug.
The prior art also discloses replacing the coiled heater wire with a spirally wound flat tape like heating element such as disclosed by Knowles in U.S. Pat. No. 4,297,785 or by Glauner et al in U.S. Pat. No. 4,211,204 in which the spiral heating element is enclosed in a tubular shield. In contrast to the helical or spiral wound heating elements, White in U.S. Pat. No. 2,178,659 discloses the use of a thin wall cylindrical heating element made from a resistance metal. The disadvantage of this type of glow plug is that it requires a relatively large electrical current to heat the thin walled cylinder to its operating temperature and because of its mass has a relatively long thermal response time. An alternate version of this type of electric heater is the gas igniter disclosed by Peri in U.S. Pat. No. 3,842,319 in which the metal cylinder is replaced by a silicon carbide cylinder. Closely spaced slots are cut through the silicon carbide cylinder to form a double helix to increase the resistance of the silicon carbide heating element.
The invention is a novel glow plug having increased resistance to the harsh environment found in the cylinder of an internal combustion engine, lower power requirements, and a thermal response time in the order of seconds rather the tens of seconds characteristic of the prior art glow plugs.